Referring to FIGS. 7A and 7B, the following description will discuss a conventional thermal mass flow meter. FIG. 7A is a cross-sectional view that shows one example of a conventional thermal mass flow meter and FIG. 7B is a graph that shows a temperature distribution on the surface of the piping of FIG. 7A. In FIG. 7B, the axis of ordinate represents the temperature, and the axis of abscissas represents the position of the piping in a flowing direction. A curved line, indicated as a dashed line, represents the temperature distribution in a state with no fluid flowing through the piping, and a curved line, indicated by a solid line, represents the temperature distribution in a state with a fluid flowing through the piping.
As shown in FIG. 7A, a heat generating element 32 is secured on the surface of a periphery of piping 30 in contact therewith, and paired temperature sensors 34 (34a, 34b), which are used for measuring the surface temperature of the piping, are placed at positions in the flowing direction of the piping 30 on the upstream side and the downstream side of the heat generating element 32, with the same distance apart therefrom. In this example, a flow-rate measuring chip 36 in which the heat generating element 32 and paired temperature sensors 34 are assembled in a single substrate by using, for example, a MEMS (Micro Electro Mechanical System) technique, and the flow-rate measuring chip 36 is attached to the piping 30 so that the flow rate in the piping 30 can be measured (for example, see Patent Document 1).
In the thermal mass flow meter, when a fluid inside piping stands still, the fluid inside the piping is heated to a predetermined temperature by the heat generating element 32, and the surface temperatures of the piping 30 at the respective positions are measured by the paired temperature sensors 34 placed with a fixed distance apart from the heat generating element 32. On the assumption that the temperature distribution of the fluid heated by the heat generating element 30 follows Gaussian distribution, when the fluid stands still, the temperatures detected by the two temperature sensors 34a and 34b are equal to each other, with the temperature difference between the two positions being zero. As shown in FIG. 7B, when the fluid flows through the piping 30, the temperature distribution is shifted toward the downstream side so that the temperatures detected by the temperature sensors 34a and 34b have a difference. The temperature distribution of the surface of the piping 30 is shifted toward the downstream side as the flow rate of the fluid flowing through the piping 30 increases; therefore, when the apex of the temperature distribution of the surface of the piping 30 is located between the temperature sensors 34a and 34b, the difference in measured temperatures of the paired temperature sensors 34 has a greater value as the flow rate of the fluid flowing through the piping 30 increases. In this manner, since there is a correlation between the flow rate of the fluid flowing through the piping 30 and the difference in measured temperatures between the paired temperature sensors 34, the flow rate of the fluid flowing through the piping 30 can be calculated by utilizing the difference in measured temperatures between the paired temperature sensors 34 based upon the correlation.
In this thermal mass flow meter using the chip 36 for use in measuring the flow rate in which the heat generating element 32 and the paired temperature sensors 34a and 34b are assembled together with each other, the paired temperature sensors 34 can be placed near the heat generating element by using the MEMS technique; therefore, even in the case where the amount of transfer of the temperature distribution is small, since the temperature sensors 34a and 34b can measure the temperatures at positions, each having an abrupt inclination, of the curved line (see FIG. 7B) indicating the temperature distribution of the temperature sensor, it becomes possible to obtain a greater value as a measured temperature difference even in the case of a fine amount of flow rate, and consequently to carry out a flow rate measuring process with high sensitivity.
Patent Document 1: U.S. Pat. No. 6,813,944